Industrial action was backed by a huge 89% on a turnout of 73% of the 110,000 balloted from members of the Communication Workers Union (CWU).

The vote passed the threshold in the Government’s controversial Trade Union Act, under which ballots need a 50% turnout for industrial action to go ahead.

The CWU believes it is a ‘watershed’ moment for unions – as well as the Royal Mail, following accusations of a ‘relentless’ programme of cost-cutting to maximise short-term profits and shareholder returns.

The union accused the company of ‘unilaterally’ closing its defined benefit, or final salary, pension scheme and there is a further dispute over pay and issues such as delivery office closures.

The union’s deputy general Secretary Terry Pullinger said: ‘This ballot result is hugely significant and demonstrates a strength of feeling that can only be translated as a massive vote of no confidence in the managerial leadership of the Royal Mail Group and the direction that they advocate.

A spokesperson told Metro.co.uk: ‘Royal Mail is very disappointed by the announcement that CWU members have voted (89% Yes) in favour of taking industrial action.

‘However, we note that with a 74 per cent turnout – and taking into account frontline employees who are not union members (18,000) – 57% have backed a strike.

‘A ballot result for industrial action does not necessarily mean there will be industrial action. Royal Mail is committed to further talks as a matter of urgency, to reach agreement with the CWU. There are no grounds for industrial action. We want to reach agreement.

‘Post Office is a separate business from Royal Mail and its employees are not involved in this dispute.’